Finding missing and illicit radioactive material and special nuclear material (SNM) is vital to such efforts as emergency response and border security. It requires a radiation detection system that can not only identify any radioisotopes present but be able to locate the radioisotopes, which may be hidden behind other materials or obstacles. Finding and identifying radioisotopes is a critical and time-sensitive task, demanding precision and speed.
Current methods for accomplishing this task all include serious drawbacks. Hand-held detectors are typically easy to operate and provide a real-time assessment of the radiation field in the surrounding area, but require a human to enter a potentially hazardous or inaccessible area to find the source. In addition, such detectors typically do not have a directional indicator and the source must be found by carrying the detector over the suspect area a number of times and observing changes in count rate.
Portal monitors are used for scanning cargo, typically on semi-trailer trucks. These monitors typically use large sets of detectors, with some sensitive to gamma rays and some sensitive to neutrons. Typical detectors include scintillators, high-purity Germanium (HPGe), and He-3 tubes. While these monitors have high efficiency, vehicles still need to drive between them relatively slowly in order for the monitors to be able to detect any hidden material. Portal monitors are also typically large in size, demand large amounts of power, and are costly to purchase and maintain. Portal monitors are also typically used as fixed devices, making any situation that could be improved by a change in geometric configuration costly at best and impractical at worst. A typical, basic portal monitor, unable to identify sources, costs in excess of one million dollars.
Position-sensitive 3D imaging detectors can also be used to locate and identify radiation sources. These devices can give an indication of the direction and distance to a source of radiation, as well as what isotopes of radiation are present. To estimate position, these systems typically rely on a detector that can record Compton scatter interactions of gamma rays in the detector bulk and reconstruct the original position of the source. While effective, this technique is slow since a large number of events must be recorded in order to obtain an accurate estimate of the location. In addition, even hand-held versions of these devices are bulky and heavy, and must again be carried into potentially hazardous areas.